1. Field of the Invention
This document relates to a liquid crystal display (LCD), and more particularly, to an LCD for compensating for a response time difference among RGB data in an LCD panel and a response time compensating method thereof.
2. Discussion of the Related Art
The use of liquid crystal display (LCDs) in various products has become more common due to favorable characteristics such as light weight, thin shape and low power consumption. LCDs are used for portable computers such as a notebook PC, automated office appliances, audio/video devices, indoor and outdoor advertisement display apparatuses, etc. LCDs control an electric field applied to a liquid crystal layer to module light emitted from a backlight unit so as to display images.
Liquid crystal has a long response time due to LCD properties such as viscosity and elasticity, as represented by expressions 1 and 2.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  Expression          ⁢                                          ⁢          1                ]            
Here, τr represents rising time when a voltage is applied to the liquid crystal, Va denotes the applied voltage, VF represents Freederick transition voltage at which liquid crystal molecules start a tilting motion, d denotes a cell gap of a liquid crystal cell, and γ represents rotational viscosity of the liquid crystal molecules.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  Expression          ⁢                                          ⁢          2                ]            
Here, τf represents falling time when the liquid crystal is restored to the original position according to elastic recovery of the liquid crystal after the voltage applied to the liquid crystal is off and K denotes modulus of elasticity of the liquid crystal.
When a data voltage VD is changed, as shown in FIG. 1, display luminance corresponding to the data voltage VD does not reach a desired luminance due to the slow response time of the liquid crystal. Consequently, the next frame of the conventional LCD is processed before a voltage charged in liquid crystal cells reaches a desired voltage, as shown in FIG. 1, and thus motion blur may generate when video data is displayed on the LCD.
To reduce the long response speed of the conventional LCD, the conventional LCD uses an over-driving (OD) compensation method that modulates a data voltage according to whether data is changed or not to increase the response time has been proposed. The conventional OD compensation method will now be described with reference to FIG. 2.
Referring to FIG. 2, the conventional OD compensation method modulates an input data voltage VD into a modulated data voltage MVD higher than the input data voltage VD and applies the modulated data voltage MVD to a liquid crystal cell such that the luminance of the liquid crystal cell can reach a target luminance MBL within a desired time. The conventional OD compensation method increases |Va2−VF2| in expression 1 based on whether data is changed in order to obtain the target luminance MBL within a single frame period. Accordingly, an LCD employing the conventional OD compensation method can compensate for long response time of liquid crystal through modulation of a data voltage to improve the picture quality of moving images. The conventional OD compensation method compares data of a previous frame with data of the current frame and sets modulation data in consideration of a variation between the data of the previous frame and the data of the current frame.
FIG. 3 is a block diagram of a conventional OD compensation circuit.
Referring to FIG. 3, the conventional OD compensation unit includes first and second frame memories 33a and 33b storing data received from a data input bus 32 and a look-up table 34 for modulating the data.
The first and second frame memories 33a and 33b alternately store data frame by frame in synchronization with a pixel clock signal and alternately output the stored data to provide previous frame data, that is, (n−1)th frame data Fn−1, to the look-up table 34.
The look-up table 34 selects previously set modulation data MRGB, as shown in Table 1, using nth frame data Fn and the (n−1)th frame data Fn−1 received from the first and second frame memories 33a and 33b as addresses to modulate the data. The look-up table 34 includes a read only memory (ROM) and a memory control circuit.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In table 1, the leftmost row represents data of the previous frame Fn−1 and the uppermost column represents data of the current frame Fn.
The nth frame data Fn is stored in the first frame memory 33a and, simultaneously, provided to the look-up table 34 in synchronization with the same pixel clock signal, as represented by a solid line in FIG. 3, during an nth frame period. Simultaneously, the second frame memory 33b supplies the (n−1)th frame data Fn−1 to the look-up table 34 during the nth frame period.
The (n+1)th frame data Fn+1 is stored in the second frame memory 33b and, simultaneously, provided to the look-up table 34 in synchronization with the same pixel clock signal, as represented by a dotted line in FIG. 3, during an (n+1)th frame period. Simultaneously, the first frame memory 33a supplies the nth frame data Fn to the look-up table 34 during the (n+1)th frame period.
Even if red (R) data, green (G) data and blue (B) data are modulated at the same OD rate in an LCD, the R data, the G data and the B data may have different response times due to a transmissivity difference and an absorption difference among RGB color filters. However, the conventional OD compensation method cannot make a) response time in an R sub-pixel, b) response time in a G sub-pixel and c) response time in a B sub-pixel equal to one another because the conventional OD compensation method modulates RGB data, as shown in FIG. 1, using the same look-up table. Consequently, although the conventional OD compensation method can increase the OD rate to improve the response time of liquid crystal, the conventional OD compensation method may cause color distortion at a boundary between a background and a moving object when color video data is displayed on an LCD due to a response time differences among RGB data.